Electron Microscopy of Protein-Nucleic Acid Complexes: Uniform Spreading of Flexible Complexes, Staining with a Uniform Thin Layer of Uranyl Acetate,

There are a number of proteins involved in DNA replication, recombination, or repair that bind stoichiometrically to single DNA strands irrespective of the nucleotide sequence, and some of these proteins also bind to single-stranded RNA. Some of the best known examples are the ssb protein of Escherichia coli , the gene 32 protein of phage T4, the gene 5 protein of the M13/fd/f1 filamentous bacterial viruses, and the more recently isolated human replication protein A (1 –4 ). Complexes formed by these proteins contain protein bound to the nucleic acid at defined ratios of the number of nucleotides per molecule of bound protein; the ratios are determined by the interactive properties of the protein. These ratios, and the structures of the complexes that are formed, may vary with factors such as changes in solution conditions that alter the binding properties of the proteins.